Airflow turbulence testing by decomposition of the wavefront of probing laser radiation by Zernike polynomials

Abstract

The results of experiments on probing the thermal airflow by laser radiation are presented. The wavefront was measured using a Shack-Hartmann sensor. One of the main requirements for the sensor during such measurements is the frequency of its operation. The frequency of 2000 Hz was used in the experiments, which made it possible to record the frequency of turbulence changes at least 200 Hz. To obtain statistical information, a sample of coordinates offset of the focal spots of the sensor was recorded. The reconstruction of the laser radiation wavefront was carried out using Zernike polynomials. As a result, a transition was made from a sample of offset coordinates of focal spots to a sample containing coefficients of expansion by Zernike polynomials. Statistical processing of the sample using the Fourier transform made it possible to determine the bandwidth occupied by each polynomial, as well as the amplitude of individual aberrations. It is shown that the calculations performed generally correspond to one of the consequences of the Taylor hypothesis - low-order aberrations are slower, but stronger compared to high-order aberrations. A graph is presented that allows us to evaluate the quality of correction of the laser radiation wavefront in the case of compensation of the first N Zernike polynomials.